It's not in the novel to Return of the Jedi, I think, but I remember it was explained years later in "Shadow of the Empire" that Obi kept some spare parts in his hut on Tatooine, which enabled Luke to build a new saber.
Obi-wan had a chest that was locked to Luke's fingerprints, knowing he would be back eventually. Inside was a book on basic force techniques, a unreadable datachips, a book on lightsaber construction, and some other crap.
Luke used some sand from outside and Obi-wan's fusion furnace and made himself a crystal, whick he cut himself. This took place during "Shadows of the Empire", between Ep V and Ep VI.
__________________ "Fear leads to Anger, Anger leads to Hate, Hate leads to Power, Power leads to Victory!" -Me
__________________ Peace is a lie
There is only passion
Through passion I gain strength
Through strength I gain power
Through power I gain victory
Through victory my chains are broken
The Force shall set me free